The present invention relates to a switching circuit for a power supply in a signal processing apparatus.
In a conventional signal processing device such as a digital audio disk player in which a signal corresponding to audio disk player in which a signal corresponding to audio and operational information is read from a digital audio disk to execute a reproducing process, a switching regulator frequently has been used as the electric power supply. Switching regulators have an advantage over some other types of power supplies in that they emit less heat. Accordingly, only a small heatsink plate is required which results in a compact electric power supply.
Referring to FIG. 1, a conventional digital audio disk player is shown. In FIG. 1, a digital disk 1 is rotated by a spindle motor 2, and recorded information is read by an opticl pickup 3. In the pickup 3, there ar included: a laser diode; an objective lens; a focus actuator; a tracking actuator; and, a photo-detector. The output signal from the pickup 3 is applied to a focus servo circuit 4, a tracking servo circuit 5, and a phase comparator 8 including at least a radio frequency amplifier. In the focus servo circuit 4, focus error is detected and a focus error signal is generated. The focus actuator installed in the pickup 3 is driven in accordance with the amount of the focus error. In the tracking servo circuit 5, a tracking error is detected and a tracking error signal is generated. The tracking error signal is sent to the tracking actuator in the pickup 3 and to a pickup feeding servo circuit 6. The pickup 3 is moved in the radial direction of the digital audio disk 1 by the output signal from the pickup feeding servo circuit 6.
A spindle servo circuit 7 receives the reproduced clock signal from the phase comparing circuit 8 and a reference clock signal from as reference clock signal generator 14 to detect a difference in phase. The spindle motor 2 is driven such that a track line velocity of the recording disk 1 becomes a constant value. The output of the phase comparing circuit 8 is applied to an Eight to Fourteen Modulation/Demodulation (EFM) circuit 9. The demodulation output of EFM circuit 9 is processed in a data processing circuit 10 in which di-interlace processing, error detection, error correction, error amendment and/or the like is effected. Then, the processed signal is temporarily stored in a data memory 11 and read out by a system clock signal with a constant period from a clock generating circuit 14 thereby to obtain an analog signal by a digital-analog (D/A) converter 12. The analog signal from the D/A converter 12 results in right and left side audio output signals through low-pass filters (LPF) 13.
On the other hand, a control signal included in the data signal from the EFM circuit 9 is supplied to a system controller 15 to read out operational data, such as an intermediate state of a musical composition, an interval state between compositions, the order number of a composition, an audio muting state and the time duration of a composition. The system controller 15 may be composed of a single micro-computer or a plurality of micro-computers. The system controller 15 supplies the operational data to a display device 18, several command signals to the pickup feeding servo circuit 6 and the spindle servo circuit 7, in accordance with the inputted information by an operational section 17. The operational section 17 comprises user controlled keys for setting a musical composition initiating time and a composition terminating time, and for setting operational modes. Numeral 16 denotes a random access memory (RAM) for storing information from the system controller 15. Numeral 19 denotes an electric power supply circuit for supplying electric power to the circuits 4 to 12 and 14 to 18.
As shown in FIG. 2, a commercial AC voltage V.sub.1 is applied to a primary coil of a transformer T in the electric power supply circuit 19. An AC voltage V.sub.2 is produced from a secondary coil of the transformer T in accordance with the ratio of coil numbers of the primary and secondary coils. The AC voltage V.sub.2 is rectified by a full wave rectifier circuit 20 and then smoothed by a smoothing capacitor C.sub.1 to obtain a non-stabilized DC voltage. The non-stabilized voltage is converted to an output voltage V.sub.0 through a transistor switching element 21 and a smoothing circuit 22 consisting of coil L and capacitor C.sub.2. The input terminal of the switching element 21 receives the output from a pulse width controlling circuit 23. The pulse width controlling circuit 23 which is, for example, comprised of a pulse width modulator, is constructed so that the pulse width controlling circuit 23 generates a serrated wave from the output pulse having a predetermined frequency produced from an oscillator24, and the serrated wave is mixed with the output voltage V.sub.o and the mixed signal is applied to a comparator (not shown) included in the pulse width modulator to compare it with a predetermined reference voltage thereby to produce an output signal corresponding to the result of the comparison. The output signal of the pulse width controlling circuit 23 has the pulse width corresponding to the output voltage V.sub.o and the switching element 21 is controlled by the pulse signal. Thus, a switching regulator is composed of the switching element 21, the smoothing circuit 22, the pulse width controlling circuit 23 and the oscillator 24. The output voltage V.sub.o is controlled so as to be a constant value by controlling the ON-duty time of the switching element 21.
In the conventional signal processing circuit as mentioned above, there are provided two oscillating devices, i.e., one is the clock generating circuit 14 and the other is the oscillator 24. The outputs of the oscillating devices have spurious radiation externally leaking through the commercial power line and the apparatus housing. The spurious radiation has frequency components due to the clock generating circuit 14, as shown by the solid lines of FIG. 3, and due to the oscillator 24, as shown by the dotted lines of FIG. 3. Thus, spurious radiation having many frequency components is undesirably generated and further the overall magnitude level of the spurious radiation is undesirably large. Therefore, it has been difficult in the conventional signal processing apparatus to eliminate the disadvantage caused by spurious radiation.